Renewable conversion of coal gangue to 13-X molecular sieve for Cd²⁺-containing wastewater adsorption performance

Using coal gangue (CG) as raw material, a new type of all solid-waste-based 13-X molecular sieve material was controllably prepared by alkali fusion-hydrothermal method. The synthetic molecular sieve was used as a solid adsorbent to treat Cd²⁺-containing wastewater, and its adsorption behavior on Cd²⁺ in aqueous solution was studied and analyzed. The microstructure and morphology of the molecular sieve were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and specific surface area analyzer. The results show that the synthesized 13-X molecular sieve has higher Brunauer–Emmett–Teller (BET) specific surface area with higher crystallinity and higher adsorption capacity for the heavy metal Cd²⁺. The adsorption process of Cd²⁺ by molecular sieve conforms to the Langmuir isotherm adsorption equation and Lagergren pseudo-second-order rate equation. Combined with thermodynamic calculation, it can be concluded that the adsorption process is physically monolayer, spontaneous and exothermic. In this study, a low-cost and naturally available synthesis method of 13-X molecular sieve is reported. Combined with its adsorption mechanism for Cd²⁺, it provides a feasible and general method for removing heavy metal ions from coal gangue and also provides a new way for the utilization of coal gangue with high added value.